The present invention relates to an exposure device for exposing an object to be exposed such as a photographic printing plate or a printed circuit board and an exposure control method when such exposure device as described above is used.
In the case of a contact exposure printing, the lines of an original pattern must be reproduced with a high degree of precision so that it is preferable to used a point light source as an exposure light source capable of emitting parallel light rays resulting in a high degree of resolution.
However, in practice, it is now impossible to obtain such ideal point light source and even if a light source substantially similar to a point light source is provided, a sufficient quantity of light for exposure cannot be obtained.
Therefore, so far a line light source such as a high-pressure mercury-vapor lamp which has some length and can emit a sufficient quantity of light for exposure has long been used as an exposure light source.
In an exposure light source utilizing such line light source, the line light source is located at the focal point of a reflecting mirror such as a parabolic mirror or a deformed elliptical mirror so that a sufficient quantity of light for exposure may be obtained.
The exposure light source utilizing a line light source can ensure a sufficient quantity of light for exposure, but since the light source is a line light source, the light rays capable of exposure are emitted from the portions of the line light source other than the center portion thereof which is located at the focal point of the reflecting mirror and there exist the light rays which arrives at the surface of an object to be exposed at random angles of incidence. As a result, the parallelism of the light rays emitted from the exposure light source is unsatisfactory so that the resolution is remarkably degraded.
Various means for correcting the parallelism have been devised and used. For instance, an optical system comprising lenses and reflecting mirrors is used. However, these means are complex in construction and need a large installation space.
As is well known in the art, an illumination distribution pattern defined by a line light source is not uniform as compared with an illumination distribution pattern defined by a point light source. That is, a non-uniform illumination distribution pattern defined by a line light source is such that the illumination is high in the direction perpendicular to the axis of the line light source while the illumination is weak in the direction of the axis of the light source. As a result, the illumination distribution pattern on a surface illuminated (to be referred as "an illuminated surface" hereinafter in this specification) by the light rays emitted from the line light source and reflected back by a reflecting mirror is not uniform so that the degree of exposure at each point on the surface of an object to be exposed; that is, the quantity of light incident at each point on the surface of the object to be exposed, is not uniform. As a result, the surface of the object to be exposed has over-exposed portions and under-exposed portions so that a large quantity of unsatisfactorily exposed objects or media is produced.